The present invention relates in general to switch-mode converters and, more particularly, to compensation of voltage loss at the output of switch-mode converters.
Switch-mode converters employing pulse width modulation (PWM) are widely used for dc-dc and dc-ac applications. In one example, a switch-mode converter is used in a dc-ac inverter configuration to produce a power-frequency sinusoidal waveform. Delays in device turn-on are present in this type of converter, and are typically of two types: (1) intentional (programmed) delays to avoid simultaneous conduction (shoot-through) of the upper and lower switching devices; and (2) delays associated with the turn-on characteristics of the switching devices and reverse recovery characteristics of the parallel diodes. These two types of delay combine to produce an effective voltage loss at the output of the converter. This voltage loss depends on switching conditions and can therefore show up in the output waveform as a notch, which causes a departure from purely sinusoidal conditions.
The notch phenomenon has been explored by others with a focus on operation of the converter at high (rated) power levels. The prior art compensates for the notching at high power levels by adding a small compensating voltage with polarity determined from measurement of the load current polarity. However, the prior art does not address aspects of the notching behavior that are seen readily only at lower converter voltage or power levels. Because the analysis presented by the prior art is incomplete, the solutions described therein are also incomplete. Solutions based solely on load current polarity are inadequate for notch correction at low converter power levels.
Thus, there is a need to compensate for notching in a way that restores a substantially pure sinusoidal output voltage waveform to the output of a converter, regardless of the load current level or power level.
The present invention is directed to systems and methods of compensating for notching in a way that restores a substantially pure sinusoidal output voltage waveform to the output of a converter, regardless of the load current level, power level, or power factor (both distortion and displacement power factor).
According to an embodiment of the invention, a method of waveform smoothing for a waveform in a switch-mode converter having a DC bus voltage and an output filter inductance comprises determining a ripple current in the converter; determining a time-to-zero; determining a turn-on time; determining the lesser of the time-to-zero and the turn-on time; determining a duty cycle modification based on the lesser of the time-to-zero and the turn-on time; and applying the duty cycle modification to the waveform.
According to another embodiment of the invention, a switch-mode converter system comprises a first half-bridge connected to an LC circuit comprising an output filter inductor and an output filter capacitor and adapted to generate a pulse width modulation (PWM) output voltage having a distorted output voltage waveform; an input voltage source connected in parallel with the first half-bridge; a bus capacitor connected in parallel with the input voltage; and a controller adapted to determine a ripple current and compensate for the distorted output voltage waveform responsive to the ripple current.
According to aspects of the invention, a second half-bridge is connected in parallel with the first half-bridge, the input voltage source is connected in parallel with the first and second half-bridges, and the second half-bridge is connected to the LC circuit. According to other aspects of the invention, the second half-bridge is connected to another LC circuit.
The foregoing and other aspects of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.